Bent vs. linear imido ligation at the octahedral molybdenum(VI) dithiocarbamate stabilised centre

Molybdenum(VI) bis(imido) complexes [Mo(NR)(2)(S2CNEt2)(2)] 3a-3p have been prepared via two synthetic routes and their structures investigated. Thermolysis of [MoO2(S2CNEt2)(2)] 1 with 2,6-disubstituted aryl isocyanates affords air-stable bis(imido) complexes 3i-3m in high yields after column chromatography, a route which fails for sterically less demanding isocyanates. A more general preparation involves room temperature addition of two equivalents of [S2CNEt2](-) to the labile bis(imido) complexes [Mo(NR)(2)Cl-2(dme)] 2a-2p (dme=1,2-dimethoxyethane) to afford 3a-3p in high yields. In this manner related complexes [Mo(NR)(2)(S2CNMe2)(2)] 4 (R=Ph or Bu-t) and [Mo(NPh)(2)(S2PPh2)(2)] 5a have also been prepared. Structural and spectroscopic studies have been carried out in order to determine the degree of bending of the imido ligands. Six complexes have been characterised by X-ray crystallography. In all cases one of the imido ligands is linear (Mo-N-C greater than or equal to 160 degrees), while the angle at the second is dependent upon the nature of R and ranges from 180 to 139 degrees. Solid-state C-13 CPMAS spectroscopy reveals that the difference in bond angles at the two nitrogen centres (Delta degrees) correlates closely with the difference in chemical shifts of the ipso-carbon atoms (Delta delta). This then has been used to probe the relative bond angles at nitrogen for non-crystallographically characterised complexes. In solution even at low temperatures both imido ligands are equivalent indicating a rapid interconversion between linear and bent bonding modes.